A dual-frequency acoustic measurement method for detecting fluid-filled channels at two boundaries in cased holes

Abstract The recognition of fluid-filled channels in cased holes is significant for the safety production of oil and gas wells. The fluid-filled channel at the cement-formation boundary is more difficult to be detected than that at the casing-cement boundary. To solve the key technical problem, a dual-frequency acoustic measurement method is proposed to obtain the cementation state of the two interfaces at the same time. The related theory is that the casing wave amplitude from the low-frequency source is sensitive to the channel at both two boundaries, but that from the high-frequency source is only sensitive to the channel at the casing-cement boundary. Therefore, high amplitudes of the casing waves from both two sources represent a channel on the casing-cement boundary, high amplitude from the low-frequency source but low amplitude from the high-frequency source represent a channel on the cement-formation boundary, and low amplitudes from both two sources represent no channel. Subsequently, numerical simulations are performed to examine the effects of several critical parameters on the dual-frequency acoustic measurement. The results show that the best combination of the measurement system is one low-frequency source at 10 kHz together with one high-frequency source at 150 kHz. For the low-frequency source, the casing wave amplitude increases obviously with the increasing channel thickness, regardless of the channel position. The increase of the cement thickness and the casing size damps out the casing wave. For the high-frequency source, the casing wave amplitude is little affected by these parameters except for the casing-cement channel. Moreover, the dual-frequency acoustic measurement method is suitable for common sandstone and shale formations. The findings of this study provide a theoretical foundation for next-generation cementing-quality-evaluation tool development.